Light interacts with materials in a variety of ways; this chapter focuses on refraction and absorption. Refraction is characterized by a material’s refractive refractive index index. We discuss some of the most useful models for the frequency dependence of the refractive index, such as those due to Cauchy, Sellmeier, Gladstone–Dale, and Wemple–DiDominico. Examples are given of the applicability of the models to actual materials. We present various mechanisms of light lightabsorption absorption, including absorption by free free carrier carriers, phonon phonons, exciton excitons and impurities. Special attention is paid to fundamental and excitonic absorption in disordered semiconductors and to absorption by rare earth, trivalent ions due to their importance in modern photonics. We also discuss the effect of an external electric field on absorption, and the Faraday effect. Practical techniques for determining the optical parameters of thin film thin films are outlined. Finally, we present a short technical classification of optical glass glasses and materials.